This invention relates to the protection against short circuit current between conductors of an electrical installation.
National and International Standards, in particular, the 15th Edition of the IEE Wiring Regulations, require that protective devices be provided in an electrical installation to break any short circuit current in conductors of each circuit of the installation before such current can cause danger due to thermal and mechanical effects produced in conductors and connections. To meet this requirement it is increasingly being specified that the prospective short circuit current at every relevant position of the complete electrical installation shall be ascertained.
In the specification of our published UK Patent Application No. 2097942A there is described and claimed a simple and inexpensive apparatus for determining the prospective short circuit current at any position in an electrical installation, which apparatus comprises means for measuring and storing the voltage at a predetermined point (e.g. the peak voltage) of a half cycle of the operating voltage between conductors at any position in an electrical installation; switching means for connection and disconnection of an impedance between said conductors to cause a current to flow therebetween for a short period; means for synchronising and sequencing operation of said voltage measuring means and actuation of said switching means; means for measuring the difference in voltage caused by said current flow; an indicating and/or recording device and, associated with said device, means for processing said voltage difference and applying it to said device for a period of sufficient length to enable the voltage difference to be displayed and/or recorded. Usually, but not necessarily, the indicating device has a scale so calibrated that a direct reading of the prospective short circuit current can be given.
In employing the apparatus of the aforesaid application to determine the prospective short circuit current at any position in an electrical installation, the value at a predetermined point (e.g. the peak value) of a half cycle of the operating voltage between conductors at the position in the electrical installation is measured and this voltage measurement is stored. A current is then caused to flow between the conductors for a short period, which current is of sufficient magnitude to effect a difference in the value of the operating voltage at the position and which voltage difference is a function of the prospective short circuit current at the position, and the voltage difference is displayed and/or recorded to provide an indication of the prospective short circuit current at the selected position in the electrical installation.